Covalent Organic Frameworks with Electron‐Rich and Electron‐Deficient Structures as Water Sensing Scaffolds

Abstract

COF by 1,3,5-tris(4-aminophenyl)benzene (TAPB) and 2,5-bis(methylthio)terephthalaldehyde (BMTTPA) to remove Hg 2+ from aqueous solutions. [27] In 2018, Thomas Bein and co-workers used 1,3,6,8-tetrakis(4aminophenyl)pyrene and thieno-[3,2-b] thiophene-2,5-dicarboxaldehyde to prepare Py-TT-COF which showed obvious color change and ultrafast response with different polarity solvents as well as vapor concentration and could be used as reversible and solid-state supramolecular solvatochromic sensors. [28] Water detection, especially the detection of trace water plays an extremely important role in ultradry reagents and experiments. Although the traditional water detection methods, such as electrochemical methods, Karl Fischer titration, and so on, have been used widely, there are still shortcomings like specialized instruments and complex operation. [29,30] Recently, luminescent-based sensors with the advantages of simple operation and low cost for water detection have been developed, for example, organic fluorophores, polymers, Cu nano clusters, and carbon dots can be used as the probe to detect water through the fluorescence change. [31] COFs with the advantages of high crystallinity, good stability, structural controllability, and especially porosity can be potential candidates for the application in water detection. But to the best of our knowledge, there are only few reports on the detection of trace amount of water in organic solvents by using COFs. For example, Yan and co-workers prepared a COF through the condensation of 4,4′,4″-(1,3,5-triazine-2,4,6triyl)trianiline (Tz) and 2,5-dihydroxyterephthalaldehyde (Da), and they used it as ratiometric sensor to detect trace amount of water in organic solvents based on the dual fluorescence emissions of the COF. [32] Herein, we synthesized thioether-based fluorescent COF with electron-rich and electron-deficient structure through Schiff-base reaction between 2,5-bis(methylthio)terephthalaldehyde (BMTA) and 1,3,5-tris-(4-aminophenyl)triazine (TAPT). The COF had good crystallinity, porosity, high chemical and thermal stability, and outstanding solvatochromic behavior. The COF could be used to detect trace amount of water in organic solvents with the low detection limit (0.12%, v/v) because of the fluorescence quenching effect caused by the interaction between water and the COF. TAPT-BMTA-COF was synthesized by Schiff-base reaction between BMTA and TAPT. (The synthetic route was shown

Covalent organic frameworks for optical applications

“…Such ICT behavior can be precisely controlled by deprotonation or protonation of specific active sites in the COF backbones. [ 114–119 ]…”

“…Such ICT behavior can be precisely controlled by deprotonation or protonation of specific active sites in the COF backbones. [114][115][116][117][118][119] Isolated ACQ building blocks in 3D framework…”

Highly Crystalline Covalent Organic Frameworks Act as a Dual-Functional Fluorescent-Sensing Platform for Myricetin and Water, and Adsorbents for Myricetin

“…Unique properties, such as large surface area, adjustable pore parameter, high crystallinity, remarkable thermal and chemical stability, etc., are the main reasons for the wide application of COFs in the fields of gas and energy storage, catalysis, and sample pretreatment . Interestingly, various COFs present high fluorescence properties owing to the extended π-conjugation system or conjugated system of hybrid elements (e.g., nitrogen element) doped in the structure. , The useful features, such as low response time, high sensitivity, high selectivity, and remarkable stability of COFs, indicate their great potential in a fluorescence-sensing application for the detection of several substances, such as metal ions, , explosives, hydrogen chloride, cations and anions, and water, and has gradually attracted increasing attention.…”

“…Because of the advantages of easy recovery, fluorescence performance not easily disturbed, good stability, good selectivity, and so on, the use of COFs for water detection has several advantages over other fluorescent materials such as fluorogenic probes, luminescent metal nanoclusters, and metal–organic frameworks . Recently, although only three kinds of fluorescent COFs have been successfully designed for the detection of water in organic solvents, ,, these worked well for water detection in organic solvents. At the same time, the shortcomings of the traditional Karl–Fischer titration method such as complex operation and requirement of highly trained operators can be effectively overcome .…”

“…20 Interestingly, various COFs present high fluorescence properties owing to the extended π-conjugation system or conjugated system of hybrid elements (e.g., nitrogen element) doped in the structure. 21,22 The useful features, such as low response time, high sensitivity, high selectivity, and remarkable stability of COFs, indicate their great potential in a fluorescence-sensing application for the detection of several substances, such as metal ions, 23,24 explosives, 25 hydrogen chloride, 26 cations and anions, 27 and water, 22 and has gradually attracted increasing attention.…”